The disclosure relates to optical displays, and more particularly to optical displays that transmit light within a predetermined viewing angle.
Conventional glass is either transparent or opaque at all angles of incidence. However, it may be desirable to have a material whose transmission properties change as a function of angle for certain applications (e.g., privacy screens, advertising, architectural glass, etc.). A privacy screen is often used in conjunction with electronic device displays (such as cell-phone displays, computer displays, etc.) and is used to restrict the field of view (i.e., viewing angle) to prevent unwanted on-lookers from viewing the display from an oblique angle. While there are conventional technologies that are under consideration for these applications, they have various drawbacks associated with them.
With respect to privacy screen applications, various polymer-based films, or multi-ply films, have been used for this purpose. In one approach, the optical screen is formed by sandwiching a light diffusing layer between various external plastic layers. The sandwich also includes adhesive layers that melt in the presence of heat and pressure to bond the multi-ply assembly together. The light diffusing polymer layer is configured to include light absorbing microlouvers that can be formed by chemical or mechanical means. While these microlouvers provide a high degree of privacy for off-axis viewing, the on-axis transmission is reduced by about 36%; and thus, one drawback associated with this approach is that it yields a relatively dark-looking display. Moreover, while the display is removable and flexible, it is easily scratched. In order to increase the hardness of the screen, another layer of a relatively hard polyethylene terephthalate (PET) material may be employed. Of course, the use of an additional layer reduces flexibility and adds cost to process.
In another approach, a polymer-film that includes light diffracting (microstructure or nanostructure) layers is being considered for architectural applications. The nanostructure layers provide anisotropic scattering of light outside a predetermined viewing angle. This approach transmits approximately 90% of the incident light at normal incidence (or within a specified viewing angle). One drawback associated with this approach relates to the “yellowing” of the film in response to being exposed to ultraviolet (UV) light. For this reason, polymer films of this type are typically limited to interior applications. Moreover, this film is also easily scratched.
In yet another approach, various thin-film formulations are being considered for window coatings in both architectural and automotive applications for energy-conscious reasons (e.g., to block solar radiation and limit the need for air conditioning). Many of these thin films comprise metallic components such as Cr, Al, Ti, and W. Reflectivity and transmission angle selectivity may be implemented by oblique-angle sputtering of these materials. One of the drawbacks to this approach is that the angular selectivity of the resultant material is rather weak. Moreover, thin films of this type typically exhibit a strong off axis color shift.
In yet another approach, a relatively expensive glass-ceramic material that includes heavy metal ingredients has been considered. In this approach, a photosensitive glass-ceramic substrate is exposed to UV light to create opal crystals embedded in the substrate. One drawback to this approach, however, relates to the heat treatment that is required to crystallize the glass-ceramic substrate. The glass ceramic substrate is disposed on a bed of CeO2 and then heated to 850° C. to prevent deformation and damage. The most obvious drawback, therefore, is that the material is prohibitively expensive from both an ingredient standpoint and a process standpoint.
In yet another brute-force energy-conscious approach, a sheet of light-diffracting spaced-apart louvers is formed and subsequently inserted between two panes of clear glass. The louvers are configured to block transmission of direct sunlight when the sun is at a predetermined angle above the horizon (or higher). One drawback associated with this approach relates to the difficulty associated with sandwiching a sheet of louvers between two sheets of glass to form a multi-ply composite structure.
The “smart window” is yet another approach that is under consideration. Smart windows typically include polymer dispersed liquid crystals, electrochromic materials or suspended particles that can be manipulated to provide opacity on demand. For example, liquid crystal smart windows are configured to switch from a light transmissive state to an opaque state in response to an applied voltage (or current). Unfortunately, this approach does not provide any angular selectivity; the display is either ON or it is OFF. Moreover, like some of the technologies considered above, the smart window approach requires a multi-layer assembly (i.e., it cannot be implemented within a single sheet of glass).